The present invention relates to a method and means for remediation of contaminated soil by biodecontamination techniques.
WO95/22374, WO95/22375 and WO95/22418 of common ownership herewith describe processes for the decontamination of material and land in-situ. Whilst these processes operate satisfactorily, they require considerable capital investment in plant and manpower and are unsuitable for larger sites needing decontamination.
It is an object of the present invention to provide an efficient and economic method and means for the biodecontamination of soil having one or more of: toxic heavy metals; metals; and, organic contaminants, without the necessity for heavy capital plant expenditure.
According to a first aspect of the present invention, there is provided a method for the remediation of soil polluted by one or more organic species and/or one or more metal species by biodecontamination techniques, the method comprising the steps of providing trough means, said trough means being impervious to acidic and alkaline liquors; providing support means at an opening of said trough means, said support means having a permeable membrane associated therewith; heaping soil to be decontaminated on top of said permeable membrane; said soil having therein sulfur oxidizing bacteria micro-organisms; optionally providing aerobic conditions and conditions to promote growth of micro-organisms to initially degrade organic contaminants within said soil heap where one or more organic contaminant species are present; maintaining said optional conditions until said one or more organic contaminants have been degraded; then, providing a supply of nutrient and liquid to said soil under conditions to promote growth of said sulfur oxidizing bacteria micro-organisms and generation of sulfuric acid to promote conversion of said one or more metal species to a sulphate; collecting a sulphate leachate in said trough means which has been provided with sulphate reducing bacteria and nutrient under conditions to promote growth of said sulphate reducing bacteria; and converting said sulphate leachate to a metal sulphide.
According to a second aspect of the present invention there is provided a composite bioreactor for the remediation of soil containing one or more organic species and/or one or more metal species, the bioreactor including a combination of a sulfur oxidizing bacteria (SOB) bioreactor directly on top of a sulphate reducing bacteria (SRB) bioreactor; the SOB bioreactor comprising a heap of said soil to be remediated and having suitable micro-organisms therein to react with and degrade contained contaminants; said SRB bioreactor comprising trough means having support means to support said soil heap associated with an opening thereof, said support means having a permeable membrane associated therewith to allow passage of liquid but substantially no soil and, sulphate reducing bacteria within said trough means; and, liquid and nutrient supply means to promote growth of said sulfur oxidizing bacteria and said sulphate reducing bacteria within said composite bioreactor.
It will be understood that the sulfur oxidizing bacteria need a source of oxidisable sulfur to provide an energy source. Where this is not already present in the soil to be decontaminated, such a source will need to be added to the soil, e.g. elemental sulfur or other sulfur compounds. Similarly, it will be understood that sulfur reducing bacteria normally require a carbon source which could be, for example, ethanol, lactate, certain organic pollutants (including, for example, volatile organic compoundsxe2x80x94VOC""s phenol(s), chlorinated aromatic compounds) or combinations thereof; where these are not present, for example due to destruction by the optional organic degradation step, they will need to be added to the leachate in the trough. The SRB reactor can thus be arranged to act to degrade polluting organic materials which have survived the SOB and any preliminary degradation within the soil heap.
In addition to the production of sulphides by the SRB, it is possible that soluble sulfur compounds will be produced, typically as sulphites. These may be returned to the SOB to provide sulfur, either directly or after a preliminary oxidation step, as appropriate for optimum operation of the bioreactor.
Preferably, the trough means may comprise a pit excavated into the ground surface and, for example, lined with a suitable impermeable plastics sheet material membrane resistant to the effects of, for example, sulphuric acid, ethanol, alkaline sulphide and hydrogen sulphide. The dimensions of the trough are dependent upon the amount of soil to be remediated and the size of the site on which the remediation is to be carried out.
Alternatively, the trough means may be constructed of concrete slabs or some other suitable cheap material on the ground surface.
The trough support means may be any suitable packing material such as rubble, pebbles, broken masonry, stones or gravel or a mixture of any or all of them for example. The support packing material may be approximately level with the ground and be of sufficient porosity to allow movement or circulation of liquor therein at least over restricted areas of the trough means.
Alternatively, the support means may comprise a metal grid spanning the opening of the trough and having a permeable membrane to substantially prevent soil from entering the trough.
Preferably, the support means also comprises or is associated with an impermeable membrane to maintain the integrity of the two reactors. When this is absent, it is possible to allow leachate to pass through the permeable membrane directly to the trough, but when it is present, it is necessary to make provision for passage of the leachate from the soil heap to the SRB, e.g. by passages through, or bypassing, the impermeable membrane, or by conduits and pumps.
The main function of the support means or packing in the SRB bioreactor is to support the soil heap in the SOB bioreactor above. It also facilitates the development and growth of an SRB biofilm.
The soil heap may be up to 2 m in height or any suitable height consistent with the width of the trough and stability of the soil heap.
The trough means and the soil heap may be provided with extraction and/or supply conduits therein to extract or supply liquids, nutrients, acids, alkalis and gases (such as air for example) as appropriate. The conduits may have suitable pump or suction means associated therewith to allow supply or extraction of liquids and/or gases. For example, the trough means may have conduits arranged to extract and pump back liquors to promote circulation and mixing thereof in the SRB bioreactor.
The SRB bioreactor may have supply/extraction conduits arranged in two or more different levels of the trough means. For example, conduits may be provided to encourage circulation/distribution of ethanol, for example, as a source of carbon for the SRB in the trough; and, the distribution of an alkali such as sodium hydroxide throughout the trough.
The SRB bioreactor will also be provided with liquid extraction conduits near to its base and near to its top for recirculation and so as to be able to take-off liquors for introduction (after treatment if necessary) into the soil heap. With respect to the lower liquid off-takes, it is advantageous if the base of the trough slopes slightly towards such offtakes.
Similarly, the soil heap of the SOB bioreactor may have supply conduits arranged therein by burying, at several different levels (optional) so as to control, for example, addition of reduced sulfur form, i.e. recycle liquor or gases from the SRB reactor. Such supplies may be provided in a continuous or discontinuous form.
A particular advantage of the bioreactor of the present invention is that the soil heap around the edges of the trough means may be used to effect a gas seal to substantially prevent hydrogen sulphide resulting from the conversion of sulphate to sulphide in the SRB bioreactor from escaping into the atmosphere. This is effected by extending the impermeable membrane inside the trough means to the outside thereof to lie flat on the ground and to have the outer periphery of the soil heap overlying the flat area. Hydrogen sulphide may be directed back into the soil heap where it reacts with other chemical constituents to provide a sulfur source to promote growth of the SOB. Thus, contamination of the atmosphere surrounding the composite bioreactor is substantially negated.
With the passage of time the organic contaminants in the soil heap are first degraded into harmless or less harmful species and the metal contaminants are eventually leached out of the soil and fed into the SRB bioreactor from where they may be retrieved in the form of a sulphide sludge and further processed for recovery or disposed of in a known manner. Once the contaminants have been removed from the soil heap it may be removed and reutilised by spreading on the land and a further batch of contaminated soil heaped onto the SRB bioreactor and processed gas above. This process may be repeated until all the soil on the site for remediation has been decontaminated. If necessary, further treatment is carried out on the decontaminated soil before it is reutilised, e.g. adjustment using lime, and/or addition of composting materials.
It will be understood that although mention has been made of a soil heap over the SRB, the composition and/or distribution of the soil may differ across the heap in a continuous or discontinuous manner, or there may be one or more further soil heaps arranged to feed the same SRB.